1. Field of the Invention
The invention relates to a method for compacting or shrinking flat glass panes, especially display glasses, in which on at least one panel at least one flat glass pane is subjected to a heat treatment in a furnace at a temperature ranging from 300xc2x0 to 900xc2x0 C.
2. Description of the Related Art
Structurally, glass is amorphous, this amorphous structure not being fixed but dependent on the prior thermal history. The amorphous structure can also change after the manufacturing process, if the glass product is subjected to a thermal stress. Associated with each change in the amorphous structure is a change in the density to higher or lower values, measured at room temperature. These structural changes and the corresponding changes in the density can be calculated at least approximately using known physical laws from the temperature-time curve of the thermal stress (George W. Scherer: Relaxation in Glass and Composites, John Wiley and Sons, Inc. (New York, Chichester, Brisbane, Toronto, Singapore), 1986, Library of Congress Catalog Card Number: 85-17871).
From the JP Abstract 08-301628 A, a thermal conduction furnace is known, in which a glass plate, which is to be heated, lies on a heating block, which is provided with an aluminum layer in order to ensure good thermal conductivity. Above the glass plate, a further heating block is disposed movably and pressed onto the glass plate. Since the heating elements are integrated into this heating block, the danger exists that, due to the different heat outputs, a different and, with that, harmful input of heat into the glass plate, which is to be treated, takes place.
The DE 3 422 347 A1 describes a method for leveling thin glasses, for which a stack of glass plates is formed, which lies on at least one layer of paper. Paper layers have also been provided between the individual glass plates. The lower layer of paper lies on a flat supporting plate, which may consist of graphite, ceramic, glass or metal. The coefficient of expansion of the supporting plate should vary in the same way as that of the thin glass, which is to be leveled, so that thick glass plates of the same material are preferred.
Aside from such leveling methods, which are carried out above the upper cooling point, there are so-called compaction methods.
During a heat treatment following the actual manufacturing process at temperatures ranging from the lower to the upper cooling point (these are the temperatures, at which the viscosity of the glass is 1014.5 dPas and 1013 dPas respectively), there is generally shrinkage of the material, which is strong at high temperatures and weak at low temperatures. It follows from the physical laws cited that the weak shrinkage at a low temperature is even weaker, if the glass previously was compacted at a higher temperature, preferably between the lower and the upper cooling points.
Flat glass, intended for the production of flat displays, generally must be compacted, so that there is no further significant shrinkage during the different temperature stresses in the course of the further manufacturing process of the display pane. If there were such further shrinkage, the different structures, applied in layers during the manufacture of the flat display, would no longer be aligned as desired.
Liquid crystal display screens (LCD""s) constitute a significant proportion of the flat displays. The usual process temperatures during their manufacture are between 200xc2x0 C. and 400xc2x0 C. The maximum permissible shrinkage of the glass substrate during the manufacturing process depends on the technology employed. For the thin film transistor (TFT) based on amorphous silicon, the shrinkage must not exceed 10 ppm (T. Yukawa, K. Taruta, Y. Shigeno, Y. Ugai, S. Matsumoto, S. Aoki (1991): Recent progress of liquid crystal display devices, In: Science and Technology of new glasses. Eds.: S. Sakka and N. Soga, pages 71-82, Tokyo, 1991).
Flat plasma displays are also widespread. Their manufacturing process comprises, for instance, the mounting of electrodes, cross members, phosphorus and dielectric layers usually at temperatures ranging from 450xc2x0 C., and 600xc2x0 C. The shrinkage of the thin glasses, used as substrate, may not exceed 20 ppm during this process.
Immediately after the glass pane is produced, for example, by a drawing or floating process, the glass generally is not yet compacted sufficiently, so that a further temperature treatment (post-annealing) must be carried out.
For example, the shrinkage of an alkali-free glass, typical of glasses used for display applications (such as AF 45 of the Deutsche Spezialglas AG, Griinenplan) during a subsequent annealing for 1 hour at 450xc2x0 C. is about 50 ppm if the glass has not previously been compacted. This shrinkage can be reduced to values less than 12 ppm by an appropriate temperature treatment. In the case of a glass with a low cooling point, (such as glass D263 of the Deutsche Spezialglas AG, Grxc3xcnenplan), the shrinkage directly after the manufacturing process is even more than 300 ppm during annealing at 450xc2x0 C. for 1 hour. This value can be reduced to less than 20 ppm by a suitable post-annealing.
The post-annealing is carried out in a batch or continuous furnace. For economic reasons, the glass panes, generally 10 to 20 panes with a thickness of the order of 1 mm, are combined into stacks. These stacks are placed onto a supporting panel and sometimes weighed down with a covering panel, for which purpose quartz plates, for example, are used.
The tendency of the stacked glass panels to adhere at higher temperatures, such as those between the lower and upper cooling point, creates difficulties. In order to avoid this adhesion of the panels, layers of inorganic powders are introduced as release agent (U.S. Pat. No. 5,073,181) between the glass panels. It is a disadvantage that the powder can affect the optical quality of the surface of the panel, if a certain particle size is exceeded.
Furthermore, the need to assure the best possible temperature homogeneity within the whole of the stack during the annealing creates difficulties. Any temperature inhomogeneity from pane to pane (that is, a vertical temperature inhomogeneity in the stack) means that, depending on the temperature program, the different panes pass through different temperature histories and, with that, have different compactions.
For a single pane, a vertical temperature gradient within the stack generally does not present a problem, since the height of the pane usually is small in relation to the height of the stack. It is different for a lateral temperature inhomogeneity. The latter means that, depending on the temperature program, the different sections of a pane pass through different temperature histories and, with that, have different compactions. For the individual pane, however, a lateral temperature inhomogeneity also means that, at the end of the annealing process, an internal stress develops in the pane, the relaxation of which during subsequent temperature stresses in turn can lead to local changes in volume. If there is a temperature gradient in the pane in the plane of the pane during the annealing, then this leads to a mismatching of the different sections of the pane during the temperature equalization at the end of the annealing process. This mismatching is compensated for by a mutual distortion of the different sections of the glass. If these tensions are relaxed during a subsequent temperature treatment, the different sections of the glass can expand or contract.
Such inhomogeneous volume expansion or shrinkage effects are a major problem for the display manufacturer, because the latter cannot compensate for them by an appropriate dimension of the masks during the subsequent coating processes.
The existence of a certain temperature inhomogeneity is unavoidable. During the heating, which necessarily is a part of the annealing process, heat must flow into the stack; during the cooling, which is also necessarily part of the annealing process, heat must flow out of the stack once again. Both processes also comprise an inner heat flow in the stack, which requires an internal temperature gradient as driving force. For geometric reasons (a typical value for the height of the stack is 2 cm; on the other hand, the lateral dimensions can be of the order of 1xc3x971 m), it is preferable to have the heat flowing in and out predominantly in a direction perpendicular to the plane of the pane. In this direction, a relatively small temperature difference is sufficient in order to attain the same temperature gradient and, with that, the same heat flow, for which a very large temperature difference would be required in the lateral direction.
A large lateral temperature difference would have two disadvantageous effects (different compaction and stress effect). On the other hand, a vertical temperature difference has only one unfavorable effect, namely the different compaction. As homogeneously as possible a temperature distribution in the lateral direction therefore is desirable. The vertical temperature difference during heating and cooling should only be as large as necessary for the flow of heat for heating or cooling of the stack. During an isothermal phase, the vertical temperature difference should also be zero.
The technical realization of annealing with a large temperature homogeneity depends on the temperature range in question. For post-annealing of display glasses, these ranges typically are 500xc2x0 C. to 700xc2x0 C. (the upper and lower cooling points of D 263 are at 529xc2x0 C. and 557xc2x0 C. respectively, the upper and lower cooling points of AF 45 are at 627xc2x0 C. and 663xc2x0 C. respectively and, for special glasses, they can also be 300xc2x0 C. to 900xc2x0 C.).
If a high degree of temperature homogeneity is required, forced air furnaces, in which the air is heated to the desired furnace temperature and circulated in the furnace, are used.
In the case of heating or cooling, the air is made a little hotter or cooler than the outer surface of the stack, in order to produce a driving force for a heat flow into or out of the stack. This temperature difference must be of the same magnitude everywhere, so that locally different heating and cooling rates and, with that, temperature inhomogeneities are not produced by local differences in the heat flow.
For several reasons, forced air heating is not desirable. First of all, additional costs are associated with the ventilation. Secondly, portions of release agents are dissolved out of the spaces between the glass panes and, under certain circumstances, dirt particles are introduced instead. This is undesirable, especially in the case of display glasses, the further processing of which usually takes place in clean rooms. Accordingly, annealing without circulating air is desirable.
U.S. Pat. No. 5,597,395 discloses a method for the compaction, in which the glass panes, in a furnace at the annealing temperature, are exposed simultaneously to a pressure all around by means of a gas.
It is an object of the invention to make available a method for the compaction of flat glass panes, which is simpler and, with that, priced more advantageously and ensures good temperature homogeneity in the glass.
This objective is accomplished by a method wherein the heat treatment is carried out in a radiation furnace, in which the flat glass pane is disposed on at least one ceramic panel with a thermal conductivity which, in the thermal treatment temperature range, is at least 5 times as great as that of the glass pane, which is to be treated.
The heat-treatment temperature or the temperature range is selected according to the glass values of the glass plate, which is to be treated, the temperatures preferably lying between the upper and lower cooling points.
In comparison to a forced air furnace, the use of a radiation furnace is more economical, because there are no additional costs for ventilation, etc. Moreover, when release agents are used, they are not dissolved out of the spaces between the glass panes.
Previously, radiation furnaces were generally not used at temperatures ranging from 300xc2x0 C. to 900xc2x0 C., because the heat transfer by radiation generally is inadequate at this temperature range for overcoming the structurally induced unevennesses in the furnace, such as the different introduction of power by heaters of apparently the same construction, uneven insulation, etc. and for producing a good temperature homogeneity in the furnace space.
Furthermore, glass has a poor thermal conductivity, typically of 1 W/(mK), as a result of which the occurrence of temperature inhomogeneities in the glass is intensified.
It has turned out that these disadvantages of a radiation furnace can be compensated for by the fact that the at least one glass pane is disposed on at least one ceramic panel with a thermal conductivity which, in the temperature range in which the heat treatment is carried out, is at least 5 times as large as that of the glass pane, which is to be treated.
The advantage of such ceramic panels consists therein that the heat flow is taken over by them and distributed simultaneously over the large surface of the glass pane, so that temperature differences in the plane of the pane are compensated for. A rapid inflow of heat during heating and a rapid outflow of heat during cooling are further advantages.
It was not possible to achieve these advantages with the quartz panels previously customarily used, because the thermal conductivity of this material only approximately corresponds to that of the glass panes to be treated.
Preferably, a stack of flat glass panes is deposited on the ceramic panel and subjected to a thermal treatment. The temperature homogeneity in the glass can be improved further if the glass pane or the stack of glass panes is disposed between two such ceramic panels.
Advantageously, pore-free ceramic panels are used because such panels cannot take up any foreign materials, such as detergent residues or the like which, during the heat treatment, could have a negative effect on the surface of the glass panels to be treated.
Preferably, ceramic panels are employed, which advantageously consist of or contain SiC. These include, for example, panels of nitride-bound SiC and silicon-infiltrated SiC, the latter material being preferred particularly because of the absence of pores.
The use of these materials as kiln furniture admittedly is known from the ceramic industry, properties such as the high strength of components, high resistance to sudden changes in temperature and great dimensional stability (no creep under thermal stress) being utilized there (A. Sonntag: xe2x80x9cImproved R-SiC Material for Cyclic Use at High Temperatures: Halsic-RXxe2x80x9d, cfi/Ber. DKG 74 (1977) No. 4, page 199). For example, plate holders for sharp firing and graining are offered (one source: AnnaWerk Keramische Betriebe, 96466 Rxc3x6dental, Material xe2x80x9crecrystallized sicxe2x80x9d).
However, in the state of the art, there are no indications that these ceramic materials are also suitable for use during the compaction of flat glass panes, because other properties of the ceramic material are in the foreground.
The use of ceramic materials has the advantage that, in comparison to a stack of glass panels, a lateral temperature difference in the furnace (that is, for example, between two opposite side walls) becomes clearly less apparent in the stack. This is surprisingly so when the heat transfer between the surfaces in the furnace, mutually irradiating one another, is low at the temperatures relevant here.
It has turned out that, due to the use of ceramic of high thermal conductivity, the lateral temperature gradient in the stack can be reduced by more than a half, when the height of the glass stack is adapted to the thickness of the two ceramic panels.
Preferably, the thickness of the ceramic panels used is such that the ratio of the total thickness of the ceramic panels to the height of the glass stack is at least 1/xcex40/W/(mK), xcex being the thermal conductivity of the ceramic material in the range of thermal treatment temperatures.